Automated software programs or bots, short for robots, are problematic for many internet services. For example, bots can be used to automatically sign up email addresses so that the addresses can be used for spam or other fraud, or to automatically purchase large blocks of tickets for concerts or shows so that the tickets can be resold at a higher price.
To distinguish software bots from human users, many internet services rely on completely automated public turing tests to tell computers and humans apart, CAPTCHAs. In an example embodiment, a CAPTCHA is a test that most humans can easily pass and that current computer programs should fail. For example, online webmail services may display CAPTCHAs after a few failed login attempts. A valid human user will be able to complete the CAPTCHA and attempt to log in again. A software bot should not be able to continue trying to break the user's password.